According to the trends toward miniaturization, multifunctionality, and decreased weight and thickness of electronic devices, a circuit substrate for use in electronic devices is required to be highly densely patterned. To this end, methods of fabricating a multilayered circuit substrate have been employed. In addition, flexible printed circuit base boards, which have flexibility such that a circuit substrate can be mounted in a narrow space, and also circuits having narrow line widths to obtain greater numbers of circuits in the same space have been used.
The typical soldering process applied to the fabrication of the multilayered circuit substrate entails environmental problems. Therefore, the fabrication of the multilayered circuit substrate requires the use of an adhesive having good adhesion, high heat resistance, and a low hygroscopic rate. However, a metallic laminate, obtained by attaching a polyimide film to a metal foil using a conventional acrylic or epoxy adhesive, is unsuitable for use in a circuit substrate requiring a multilayered structure, flexibility, good adhesion, and high heat resistance. Hence, a 2-layer copper clad laminate (2CCL) type flexible metallic laminate, resulting from direct attachment of polyimide and a metal foil without the use of an adhesive, has been developed. As such, since the metallic laminate thus obtained is advantageous because it has thermal stability, durability, and electrical properties superior to those of 3CCL (3-layer Copper Clad Laminate), comprising a metal layer and a polyimide film attached thereto using a conventional adhesive, it is thus suitable for use in a flexible circuit substrate.
Further, in order to realize a highly dense printed circuit board, the line width thereof should be further decreased. Accordingly, in the 2CCL type metallic laminate, the metal layer is formed to be thinner and the polyimide layer is formed to be thicker so as to have good processibility and insulation properties.
However, in the case where the polyimide layer is formed to a predetermined thickness or greater, a lot of foam may be undesirably formed on the surface of the polyimide layer when curing the polyimide layer on the metal layer.